1. Field of the Invention
This invention relates generally to the electronic measurement of crystal resonator temperature anomalies and more particularly to the frequency or admittance-temperature anomalies occurring in quartz crystal resonators.
2. Description of the Prior Art
Because of precision frequency control requirements for digital communication and position location systems currently in use, it is imperative that the crystal resonators utilized have very smooth and reproducible frequency-temperature characteristics. One reason why the characteristic is required to be smooth is because the temperature compensating network used in temperature compensated crystal oscillators must be capable of compensating for the usual "S" shaped curve of an AT-cut crystal, for example. Consequently, it is the usual practice to individually test each crystal resonator in an oven over a selected temperature range and monitor the oscillator frequency on a continuous basis. Such temperature runs require expensive apparatus and take considerable time for a careful scan. If the crystal is acceptable, the frequency-temperature characteristic will exhibit a substantially smooth "S" curve. If on the other hand the crystal has an anomaly in its frequency-or admittance-temperature characteristic, referred to as an "activity dip" or "band break," the curve exhibits an abrupt change or discontinuity. Either the frequency or resistance curve can be used as an indication of the presence of such an anomaly depending upon where on the "S" curve the "activity dip" occurs.
These anomalies are generally considered to be caused by various combinations of different modal frequencies coming into coincidence at particular temperatures because of differing temperature coefficients. At least two distinct species of activity dips exist; one being "design-related" dips, while the other is termed "process-related" dips. The latter type arises from shortcomings in the processing phases, where for example, an improperly deposited electrode film peels or blisters in a reversible, temperature-dependent manner. Design-related activity dips are those for which structural configuration remains unaffected by temperature changes, but instead depends solely upon geometry and material constants. It is these latter type anomalies to which the present invention is directed. The presence of design-related activity dips is a persistent problem and as a consequence, necessitates a great deal of costly testing for medium and high precision resonator units in the manner mentioned above, i.e., temperature runs. Doubly rotated cuts generally could be expected to have even more problems in this regard than AT-cut crystals, since they exhibit less symmetry and have therefore a more complicated mode spectrum when lateral boundaries are taken into account. The exception appears to be the SC/TTC orientation for which activity dips have yet to be encountered.
The modal interference that takes place can be either linear or non-linear. If the impressed voltage is capable of driving the desired thickness mode and at the same time drive a harmonic effectual mode, for example, then the vibrator admittance will reflect this fact as the linear superposition of the separate modal admittances. With temperature changes, it is possible for the two resonance frequencies to cross and produce an anomaly. Such linear activity dips have been described by several investigators in the following publications: "Activity Dips in AT-Cut Crystals," A. Wood, et al., Proceedings 21st Annual Frequency Control Symposium (AFCS), Ft. Monmouth, N.J., April, 1967, pp. 420-435, and "The Unwanted Responses of Crystal Oscillator Controlled by AT-Cut Plate," H. Fukuyo, et al., Bulletin Tokyo Inst. Tech., No. 82, September, 1967, pp. 53-64; Proceedings 21st AFCS, April, 1967, pp. 402-419.
Non-linear activity dips are less well understood and perhaps more important. The Wood, et al. publication also found that the AT-Cut fundamental thickness shear frequency to be effected by interfering modes at twice its frequency. In a publication entitled "On Activity Dips of AT Crystals At High Levels of Drive," C. Franx, Proceedings 21st AFCS, April, 1967, pp. 436-454, the same type of coupling due to a mode at three times the fundamental frequency was reported. Similar results were obtained and reported by the aforementioned Fukuyo reference. In all cases, the sensitivity of mode coupling to power levels is a characteristic of non-linearity.
It is an object therefore of the present invention to provide a new and improved method and apparatus for testing for activity dips in piezoelectric (quartz) resonators which obviates the heretofore required temperature runs.